1. Field of the Invention
The invention relates generally to an air bubble purging system for filters and, more particularly, to an air bubble purging system for photoresist filters, the system having a venturi valve for creating a vacuum that aids in the purging of the air bubbles out of the photoresist filter.
2. Description of the Related Art
A photoresist filter is used in semiconductor wafer processing. The photoresist filter is used to collect impurities in the photoresist, which is typically in liquid form, before the photoresist is coated on the semiconductor wafer during a photolithography process. Impurities in the photoresist will otherwise cause defects in the processed semiconductor wafer.
Generally, the photoresist filter is replaced as part of periodic maintenance. During replacement, air bubbles are introduced and trapped within the filter. The air bubbles, if not removed, become suspended in the photoresist and will cause defects in the processed semiconductor wafer.
For this reason, it is necessary to purge the air bubbles out of the filter after filter replacement. A conventional air bubble purging system for photoresist filters is illustrated in FIG. 1. The system includes a filter housing 10 having a photoresist inlet 20, a photoresist outlet 30, and a drain line 40. The drain line 40 is connected to a common drain which is open to ambient pressure. The drain line 40 has disposed therein a shut-off valve 45 which is opened during the purging process.
Air bubbles are purged by opening the shut-off valve 45 and pumping the photoresist through the filter. Air bubbles rise to the top of the filter housing 10 with each pump stroke and are pushed out through the drain line 40. Periodically, the photoresist is coated onto a semiconductor wafer and defect monitors are used to examine the quality of the coated photoresist layer. If it is determined that the quality of the coated photoresist layer is unacceptable, the shut-off valve is kept open and more photoresist is pumped through the filter to cause additional air bubbles to be purged from the filter. This process is repeated until a coated photoresist layer of acceptable quality can be seen on the defect monitors.
The invention provides an air bubble purging system for filters that is less time consuming and wastes comparatively very little photoresist. The invention employs a venturi valve to create a vacuum to attract air bubbles to the top of the filter housing so that air bubbles can be quickly removed from the filter, requiring less pumping and waste of photoresist and faster qualification of the photoresist. In the invention, the venturi valve is disposed in the drain line of the filter and connected to a high pressure source line so that when the high pressure source is released to atmosphere and a high rate of flow appears in the high pressure source line, vacuum is created in the drain line by the venturi effect. The vacuum, in turn, suctions the air bubbles to the top surface of the filter, for easier purging.
Additional objects, features and advantages of the invention will be set forth in the description of preferred embodiments which follows.